include more details about the kubeadm upgrade backup process

during upgrade if the process fails on static pods, kubeadm writes a folder called `/etc/kubernetes/backup`.

currently we do not:
- document how this folder works. **WIP**
- have test coverage when kubeadm falls back to a manifest in the backup folder. **EDIT: we have some unit tests for this.**

xref: https://github.com/kubernetes/kubeadm/issues/1617#issuecomment-515967794
